Abstract

Alexander Island, situated off the west coast of the Antarctic Peninsula, contains a suite of Late Cretaceous to Early Tertiary subduction-related magmatic rocks. The rocks occupy a fore-arc position 100–200 km trenchward of the main arc (Antarctic Peninsula) and they become younger northward along the length of the island. Major and trace element geochemistry for 222 samples shows the suite to be a medium to high-K calc-alkaline series, ranging in composition from picro-basalt to rhyolite. Andesite samples show a large range in MgO and Mg#, with nine samples representing high-magnesian andesites.Sr and Nd isotopic data indicate that the andesites range isotopically to more depleted mantle compositions than the associated basalts. The dacite/rhyolites can be related compositionally to the andesites by assimilation of typical Pacific rim accretionary material. To produce high-magnesian andesite lavas, it is necessary to introduce a suitable source of heat into the fore-arc, thus enabling partial melting of depleted sub fore-arc hydrous peridotite. A causative link with ridge subduction prior to magmatism is proposed, with successive ridge–trench collisions producing a temporal migration of the magmatism and high geothermal gradients in an anomalously hot fore-arc region.

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